The present invention relates to sheathed wires and cables, and more particularly to wires and cables with polyethylene resin sheaths having excellent stress crack resistance, abrasion resistance and impact resistance.
As materials of sheaths for protecting wires (wire sheaths as the outermost layers) and those for protecting power or telegraph cables, synthetic resins such as polyethylene and polyvinyl chloride have heretofore been employed.
Such sheaths need to be excellent in properties such as stress crack resistance (ESCR), abrasion resistance and impact resistance, particularly low-temperature impact resistance, differently from coatings for directly insulating individual wires or cables. As their uses are diversified and the use conditions become severer recently, development of sheathed wires and cables having better properties than before have been more desired.
It is an object of the present invention to provide wires and cables with polyethylene resin sheaths more improved in stress crack resistance, abrasion resistance and impact resistance than the conventional polyethylene sheaths.
The sheathed wire or cable is obtained by coating an outermost layer of a wire or cable with a polyethylene resin (A) produced by polymerization using a single-site catalyst.
In the present-invention, the polyethylene resin (A) is preferably a copolymer of ethylene and an xcex1-olefin of 3 to 20 carbon atoms and has the following properties:
(1) the density (d) is in the range of 0.880 to 0.950 g/cm3, and
(2) the melt flow rate (MFR, ASTM D 1238, 190xc2x0 C., load: 2.16 kg) is in the range of 0.01 to 20 g/10 min.
The polyethylene resin (A) preferably further has the following properties:
the n-decane-soluble component fraction (W (% by weight)) at room temperature and the density (d (g/cm3)) satisfy the following relation
in case of MFRxe2x89xa610 g/10 min:
W less than 80xc3x97exp(xe2x88x92100(dxe2x88x920.88))+0.1,
in case of MFR greater than 10 g/10 min:
W less than 80xc3x97(MFRxe2x88x929)0.35xc3x97exp(xe2x88x92100(dxe2x88x920.88))+0.1.
The polyethylene resin (A) preferably furthermore has the following properties:
the flow index (FI (1/sec)), which is defined as a shear rate at which the shear stress of the polymer in a molten state at 190xc2x0 C. reaches 2.4xc3x97106 dyne/cm2, and the melt flow rate (MFR (g/10 min)) satisfy the following relation
FI greater than 75xc3x97MFR.
Moreover, it is preferable that when the polyethylene resin (A) is subjected to a temperature rise elution test (TREF), a component that is eluted at a temperature of not lower than 100xc2x0 C. is present in the resin, and the amount of the component is not more than 10% by weight of the whole elution amount.
In the polyethylene resin (A), high-pressure low-density polyethylene (B) may be contained in an amount of not more than 50% by weight.
In the present invention, the polyethylene resin (A) preferably has the following properties:
(i) the 50% crack occurrence time (F50, ASTM D 1698) that becomes an indication of stress crack resistance is not less than 600 hours,
(ii) the abrasion wear as measured by a Taber abrasion test (JIS K 7204, load: 1 kg, truck wheel: CS-17, 60 rpm, 1000 times) is not more than 10 mg, and
(iii) the Izod impact strength (ASTM D 256, notched) as measured at xe2x88x9240xc2x0 C. is not less than 40 J/m2.
The sheathed wire and cable according to the invention are described in detail hereinafter.
The polyethylene resin for forming sheaths of the sheathed wire and cable according to the invention is a polyethylene resin (A) having specific properties and is prepared by the use of a single-site catalyst such as a hitherto known metallocene catalyst or Brookhart catalyst. In the polyethylene resin (A), high-pressure low-density polyethylene (B) may be contained.
The polyethylene resin (A) for use in the invention has a density (ASTM D 1505) of usually 0.880 to 0.950 g/cm3, preferably 0.885 to 0.940 g/cm3, more preferably 0.890 to 0.935 g/cm3. When the polyethylene resin (A) having a density in the above range is used, a wire sheath and a cable sheath having excellent abrasion resistance and flexibility can be formed.
The density is determined as follows. Strands obtained in the measurement of melt flow rate (MFR) at 190xc2x0 C. under a load of 2.16 kg is heat treated at 120xc2x0 C. for 1 hour and slowly cooled to room temperature over a period of 1 hour. Then, the density is measured by a density gradient tube.
The polyethylene resin (A) has a melt flow rate (MFR, ASTM D 1238, 190xc2x0 C., load: 2.16 kg) of usually 0.01 to 20 g/10 min, preferably 0.03 to 15 g/10 min, more preferably 0.05 to 10 g/10 min.
The n-decane-soluble component fraction (W (% by weight)) in the polyethylene resin (A) for use in the invention at room temperature and the density (d (g/cm3)) of the resin satisfy the following relation
in case of MFR  less than 10 g/10 min:
W less than 80xc3x97exp(xe2x88x92100(dxe2x88x920.88))+0.1,
preferably W less than 60xc3x97exp(xe2x88x92100(dxe2x88x920.88))+0.1,
more preferably W less than 40xc3x97exp(xe2x88x92100(dxe2x88x920.88))+0.1,
in case of MFR greater than 10 g/10 min:
W less than 80xc3x97(MFRxe2x88x929)0.35xc3x97exp(xe2x88x92100(dxe2x88x920.88))+0.1.
It can be said that such an ethylene copolymer has a narrow composition distribution.
The n-decane-soluble component fraction (W) in the polyethylene resin (A) at room temperature is desired to be not more than 3% by weight, preferably not more than 2% by weight. When the n-decane-soluble component fraction (W) is not more than 3% by weight, a sheath that is free from surface tackiness when exposed to high temperatures can be obtained.
The n-decane-soluble component fraction (W) at room temperature is determined by completely dissolving 0.5 g of a polyethylene resin in 500 ml of n-decane under reflux at a boiling point of n-decane, then cooling the resulting solution to room temperature (25xc2x0 C.), filtering the solution, evaporating n-decane from the filtrate, and calculating a weight ratio of the residue to the initial polyethylene resin.
The flow index (FI (1/sec)) of the polyethylene resin (A) for use in the invention, which is defined as a shear rate at which the stress of the polymer in a molten state at 190xc2x0 C. reaches 2.4xc3x97106 dyne/cm2, and the melt flow rate (MFR (g/10 min)) of the resin satisfy the following relation
FI greater than 75xc3x97MFR,
preferably FI greater than 80xc3x97MFR,
more preferably FI greater than 85xc3x97MFR.
The flow index (FI) is determined by extruding a resin from a capillary with changing a shear rate and measuring a shear rate corresponding to the given stress. That is, this measurement was carried out using the same sample as in the MT measurement and using a capillary type flow property tester manufactured by Toyo Seiki Seisakusho K. K. under the conditions of a resin temperature of 190xc2x0 C. and a shear stress range of about 5xc3x97104 to 3xc3x97106 dyne/cm2. In this measurement, the diameter of a nozzle is changed as follows according to the MFR (g/10 min) of the resin to be measured.
MFR greater than 20: 0.5 mm
20xe2x89xa7MFR greater than 3: 1.0 mm
3xe2x89xa7MFR greater than 0.8: 2.0 mm
0.8xe2x89xa7MFR: 3.0 mm
When a polyethylene resin having a narrow composition distribution is prepared by the prior art technique, also the molecular weight distribution is usually narrowed, resulting in bad flowability (moldability) and small FI. When the polyethylene resin (A) for use in the invention has the above relation between. FI and MFR, a low stress can be retained even in the high shear rate region and the moldability becomes better.
When the polyethylene resin (A) for use in the invention is subjected to a temperature rise elution test (TREF), a component that is eluted at a temperature of not lower than 100xc2x0 C. is desired to be present and the amount of the component is preferably not more than 10% of the whole elution amount. The component that is eluted at a temperature of not lower than 100xc2x0 C. is a highly crystalline high-density component. If the amount of this high-density component increases, the heat resistance is improved, but if the amount thereof exceeds 10% by weight, the flexibility of the polymer is lowered, and such a polymer is unfavorable as a material of sheath.
The temperature rise elution test (TREF) is carried out in the following manner.
A sample solution is placed in a column at 140xc2x0 C., then cooled to 25xc2x0 C. at a cooling rate of 10xc2x0 C./hr and heated at a heating rate of 15xc2x0 C./hr to detect, on the online system, components having been continuously eluted at a constant flow rate of 1.0 ml. This test was carried out using a column of 2.14 cm (diameter)xc3x9715 cm, glass beads having a diameter of 100 xcexcm as packing and orthodichlorobenzene as a solvent under the conditions of a sample concentration of 200 mg/40 ml-orthodichlorobenzene and a pour of 7.5 ml.
The polyethylene resin (A) can be prepared by polymerizing ethylene only or copolymerizing ethylene and an xcex1-olefin of 3 to 20 carbon atoms in the presence of a single-site catalyst, e.g., a so-called metallocene type olefin polymerization catalyst containing a metallocene catalyst component described in Japanese Patent Laid-Open Publications No. 9724/1994, No. 136195/1994, No. 136196/1994, No. 207057/1994, etc.
That is, the polyethylene resin (A) for use in the invention is an ethylene homopolymer or an ethylene/xcex1-olefin copolymer prepared by the use of a single-site catalyst such as a metallocene type olefin polymerization catalyst.
Examples of the xcex1-olefins of 3 to 20 carbon atoms employable in the copolymerization with ethylene include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene and 1-dodecene. Of these, preferable are xcex1-olefins of 3 to 10 carbon atoms, and particularly preferable are xcex1-olefins of 4 to 8 carbon atoms.
These xcex1-olefins can be used singly or in combination of two or more kinds.
In the polyethylene resin (A), constituent units derived from ethylene are desirably present in amounts of not less than 75% by weight and less than 100% by weight, preferably 80 to 99% by weight, more preferably 80 to 97% by weight, and constituent units derived from an xcex1-olefin of 3 to 20 carbon atoms are desirably present in amounts of not more than 25% by weight, preferably 1 to 20% by weight, more preferably 3 to 20% by weight.
In the present invention, the polyethylene resin (A) can be used singly as a polyethylene resin for sheath, or it may be used as its blend with high-pressure low-density polyethylene (B). When the polyethylene resin (A) is used as its blend with the high-pressure low-density polyethylene (B), the high-pressure high-density polyethylene (B) is used in an amount of not more than 100 parts by weight, preferably not more than 70 parts by weight, more preferably not more than 50 parts by weight, based on 100 parts by weight of the polyethylene resin (A). When the polyethylene resin (A) is used in this proportion, a sheath having excellent stress crack resistance, abrasion resistance and low-temperature impact resistance can be formed.
In the present invention, the polyethylene resin (A) can be used singly or after blended with a resin having different melt flow rate and density.
As the polyethylene resin (A), particularly preferable is such a polyethylene resin as to form a sheath having the following properties:
(i) the 50% crack occurrence time (F50, ASTM D 1698) that becomes an indication of stress crack resistance of the sheath is preferably not less than 600 hours, more preferably not less than 1000 hours,
(ii) the abrasion wear as measured by a Taber abrasion test (JIS K 7204, load: 1 kg, truck wheel: CS-17, 60 rpm, 1000 times) is preferably not more than 10 mg, more preferably not more than 8 mg, and
(iii) the Izod impact strength (ASTM D 256, notched) as measured at xe2x88x9240xc2x0 C. is preferably not less than 40 J/m2, more preferably not less than 50 g/m2.
The high-pressure low-density polyethylene (B) that is optionally used in the invention is polyethylene prepared by polymerizing ethylene under high pressure in the presence of a radical polymerization catalyst, and may be one obtained by copolymerizing ethylene and a small amount of a vinyl monomer if necessary.
The high-pressure low-density polyethylene (B) has a density (ASTM D 1505) of usually not more than 0.930 g/cm3, preferably 0.910 to 0.925 g/cm3. When the high-pressure low-density polyethylene (B) having a density in the above range is used, a polyethylene resin capable of forming a sheath of excellent abrasion resistance and flexibility can be obtained. The density can be measured in the same manner as described above.
The high-pressure low-density polyethylene (B) has a melt flow rate (MFR, ASTM D 1238, 190xc2x0 C., load: 2.16 kg) of usually 0.05 to 20 g/10 min, preferably 0.1 to 10 g/10 min. When the high-pressure low-density polyethylene (B) having a melt flow rate in the above range is used, extrusion coating processability can be enhanced.
In the polyethylene resin for sheath employable in the invention, hitherto known additives, such as heat stabilizer, weathering stabilizer, carbon black, pigment, flame retardant and anti-aging agent, may be contained in amounts not detrimental to the object of the invention, in addition to the polyethylene resin (A) or the polyethylene resin (A) and the high-pressure low-density polyethylene (B).
The wire and cable with sheaths having the above properties can be formed by a conventional extrusion coating method using the polyethylene resin (A) or its blend with the high-pressure low-density polyethylene (B).
The sheathed wire and cable according to the invention exhibit excellent stress crack resistance, abrasion resistance and low-temperature impact resistance because their sheaths have stress crack resistance, abrasion resistance and low-temperature impact resistance better than those of conventional polyethylene sheaths.